Synthetic fibers are used in a wide variety of textile applications including clothing and other fabric items which, although desirably white or natural in color in many circumstances, are also desirably manufactured and marketed in a variety of colors and patterns in other circumstances.
As known to those familiar with the textile arts, several techniques are used to add color to textile products. In general, these techniques add such color to the basic structures of textile products: fibers, yarns made from fibers, and fabrics made from yarns. Thus, certain techniques dye individual fibers before they are formed into yarns, other techniques dye yarns before they are formed into fabrics, and yet other techniques dye woven or knitted fabrics.
Particular advantages and disadvantages are associated with the choice of each coloring technique. Some exemplary definitions and explanations about dyes and coloring techniques are set forth in the Dictionary of Fiber & Textile Technology (1990), published by Hoechst-Celanese Corporation, on pages 50-54.
Although the term "dye" is often used in a generic sense, those familiar with textile processes recognize that the term "dye" most properly describes a colorant that is soluble in the material being colored, and that the term "pigment" should be used to describe insoluble colorants.
Because polyester, particularly polyethelene terephthalate ("PET"), is so widely used in textile applications, a correspondingly wide set of needs exist to dye polyester as filament, yarn, or fabric. Although coloring yarns and fabrics are advantageous or desirable under some circumstances, coloring the initial fiber offers certain performance benefits such as improved fastness. As an additional and increasingly important consideration, coloring filament rather than yarns and fabrics tends to reduce secondary effects that must be dealt with to prevent air and water pollution that would otherwise be associated with various coloring processes.
Conventionally, a "masterbatch" approach has been used to color fibers (or filaments) during the melt spinning process. As known to those familiar with this technique, in the masterbatch process, the desired colorant is dispersed at a relatively highly concentrated level within a carrier polymer. In a following process step, the masterbatch of highly concentrated colored polymer is introduced to the melt spinning system of the polymer and blended with virgin polymer at a ratio that hopefully achieves the desired color.
Condensation polymers, however, offer particular challenges to the masterbatch system. As is known to those familiar with chemical reactions, a condensation polymer results from a reaction in which two monomers or oligomers react to form a polymer and water molecule. Because such reactions produce water, they are referred to as "condensation" reactions. Because of chemical equilibrium, however, the water must be continually removed from the polycondensation reaction, otherwise it tends to drive the reaction in the other direction; i.e. depolymerize the polymer. This results in a loss of molecular weight in the polymer which is referred to as hydrolytic degradation. In particular the molecular weight (measured by the intrinsic viscosity or "IV") of polyester can easily be decreased by as much as 0.15 dl/g (0.55-0.75 dl/g is considered a good viscosity for filament). As a greater problem--and one that becomes evident during later processing of filament and yarn--the loss in IV is quite variable depending upon the quality of process control of the masterbatch drying and extrusion systems. In particular, obtaining the required color specification of the masterbatch chip sometimes requires re-extruding the polymer to obtain a desired color correction. Unfortunately, such re-extrusion for color matching purposes tends to increase the loss in molecular weight even further.
Masterbatch "chip" is generally introduced into the spinning process using several options each of which tends to provide an extra source of variation for the resulting molecular weight. Because there are several process steps during which molecular weight can be lost, the effect tends to be cumulative and significant. The overall effect is a significant reduction in the molecular weight of the filament that manifests itself as an orientation variability in the resulting yarn. In turn, the orientation variability produces a resulting variability in the physical properties of the yarn such as elongation, tenacity, and draw force.
Such variability in the physical properties of spun yarn generates several additional problems. For example, partially oriented yarn (POY) which is draw textured must exhibit uniform draw force to assure that its preaggregate tension stays within desired specifications. If the yarn properties are outside of such specifications, various problems such as twist surging occur and prevent processing the yarn at commercial speeds. Furthermore, the drawing performance of spun yarns, whether POY, low orientation yarns (LOY), fully oriented yarns (FOY), or staple, is highly dependent upon consistent elongation because the imposed draw ratio cannot exceed the inherent drawability of the spun yarn (as measured by the elongation). Additionally, consistent physical properties of the final drawn or draw textured filament are desirable for optimum performance of fabrics and other end-use products.
In a practical sense, the variation in physical properties from filament to filament, fiber to fiber, and yarn to yarn forces the various textile manufacturing processes and machinery to be continually readjusted whenever a new colored fiber or yarn is introduced. Thus, the problems inherent in masterbatch coloring tend to raise the cost and lower the productivity of later textile processes that incorporate masterbatch colored fibers and yarns.